Monday Pharmaceutical Mystery: November 11

What is causing a positive drug test in a patient that only takes blood pressure medications?

A frazzled, upset, 45-year-old female patient approaches you at the consultation window. She tells you that she was just fired from her job because of a positive drug test. She says that she has never taken anything except her regular blood pressure medications (labetalol and hydrochlorothiazide).

She wants to buy a drug test at the pharmacy to see the results for herself. She wants you to recommend a drug test kit. But the question remains, why is her test showing up positive?

Mystery: Why is this patient’s drug test positive, despite only taking blood pressure medications?

Solution: Labetalol can cause a false positive drug test.

Labetalol has been reported to show a false positive result for amphetamines in urine screenings when certain tests are used. The package insert recommends “when patients being treated with labetalol have a positive urine test for amphetamine using these techniques, confirmation should be made by using more specific methods, such as a gas chromatographic-mass spectrometer technique.” 1

Other drugs that can show up as a false positive for amphetamines, including bupropion, amantadine, fluoxetine, phentermine, ranitidine, and trazodone.2

REFERENCES

  • Labetalol package insert https://www.accessdata.fda.gov/drugsatfda_docs/label/2010/018716s026lbl.pdf Accessed November 1, 2019
  • Urine Drug Screening: A Valuable Office Procedure. https://www.aafp.org/afp/2010/0301/p635.pdf Accessed November 1, 2019